Device for increasing the steering effect of a rudder

ABSTRACT

An apparatus for enhancing the steering effect of ship rudders comprising a rudder with at least one passageway for supplying seawater to ducts in the rudder, and a profile element extending substantially vertically at a leading edge of the rudder and being pivotable about a generally vertical axis, wherein distribution ducts formed between the profile element and the rudder communicate with the passageway and are adjustable so that relative movement of the profile element and the rudder causes seawater to be discharged along the rudder surface facing abaft.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an arrangement for enhancing the steeringeffect of ships' rudders, comprising at least one passageway forsupplying seawater from a pump to ducts in the rudder, and meanscommunicating with the ducts and being adapted to discharge seawatersupplied through them, at the rudder surface.

2. Description of the Related Art

Previously there are known many forms of steering effect enhancing meansor elements in connection with ships' rudders. In particular atrelatively low speeds such enhanced steering effect can be very usefulwhen manoeuvring a ship.

A known form of such means are flaps or fins being hinged or pivotablein relation to the main part of the rudder, in particular being locatedat the trailing edge thereof. An example of such a design is to be foundin Norwegian Patent 160.126.

Another form of auxiliary means are ports or nozzles in or at the ruddersurface and being adapted to discharge water flows that can contributeto enhancing the steering effect of the rudder. Examples of such designscan be found in British patent 1.309.387 and Japanese patentpublications Nos. 57.110.597, 57.110.598 and 57.121.998. These variousdesigns are in part based on the supply of seawater to the rudder andits discharge openings or nozzles, through the actual rudder stock.However, there are also described designs where the seawater inlet andthe transfer to the ports or openings takes place in a passive manner,i.e. without the influence of any pump which can impart energy to theseawater flow in the form of an increase pressure or velocity.

SUMMARY OF THE INVENTION

The present invention is directed to an arrangement for such enhancedsteering effect of ship rudders, with an improved efficiency and in parta simplification of the arrangement compared to the previously knowndesigns. In this connection it is a substantial point in the basicsolution according to the invention, that there is produced anadditional lift as a result of extra water flow supply oversubstantially the whole of one rudder side or rudder surface when therudder is deflected at an angle so as to manoeuvre the ship to thestarboard or port side. It is essential in this connection that suchincreased lift is generated at the rudder surface facing abaft with therudder deflection concerned, for the desired turning movement of theship.

Closer statements of the novel and specific features of the arrangementaccording to the invention, are found in the claims.

An apparatus according to the invention comprises a rudder with at leastone passageway for supplying seawater to ducts in the rudder, and aprofile element extending substantially vertically at a leading edge ofthe rudder and being pivotable about a generally vertical axis, whereindistribution ducts formed between the profile element and the ruddercommunicate with the passageway and are adjustable so that relativemovement of the profile element and the rudder causes seawater to bedischarged along the rudder surface facing abaft.

The apparatus may further comprise a turning mechanism associated with arudder stock, wherein the profile element is angularly adjustable aboutthe vertical axis by the turning mechanism, the turning mechanismcomprising at least one manoeuvre rod, one end of which is directly orindirectly articulated to the profile element.

The profile element may comprise internal connection ducts which form acentral duct that leads out to either side of the profile element fordischarging seawater adjacent to outer portions of the profile elementand distributes seawater to either one or the other side depending uponan angular position of the profile element with respect to the verticalaxis.

The apparatus may further comprise a control valve for controllingdistribution of seawater to either one or the other side of the profileelement, the control valve comprising first and second tubular elements,the tubular elements being concentric about the vertical axis, the firstelement being rotationally connected to the rudder, the second elementbeing rotationally connected to the profile element, and both elementshaving respective pairs of cooperating, axially extending slot openingsforming substantially open and substantially closed flow openingsdepending upon the angular position of the profile element, whereby eachof the slot openings in the second tubular element communicates with aseparate one of the distribution ducts.

A space between the leading edge of the rudder and the inside of theprofile element may constitute the distribution ducts.

The apparatus may further comprise a nozzle with an adjustable angleprovided at a trailing edge of the rudder and adapted to dischargeseawater supplied through an associated duct, through the rudder. Thenozzle may be incorporated in a rear, pivotable rudder fin.

The passageway may have, in part, the form of an annular spacesurrounding a rudder stock and a swivel device may be provided for arotational connection between a ship hull and the rudder.

The annular space may be subdivided into two passages, of which onecommunicates with a duct leading to the nozzle, and the othercommunicates with a duct leading to the profile element.

The passageway may extend in part internally in a rudder stock and adistribution valve for supplying water to the profile element and thenozzle, may be arranged axially in the extension of the rudder stock.

The distribution valve may comprise a rotatable valve body which,depending upon its rotational position, is adapted to substantiallyblock at least one of two valve openings that communicate with a duct tothe nozzle on one hand, and the distribution ducts of the profileelement on the other hand.

In the following description the invention will be explained moreclosely with reference to the drawings, whereby also various specificfeatures and advantages will appear from the description with referenceto the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

An overview of the figures of drawings:

FIG. 1 in elevation and partial section shows a first embodiment of therudder arrangement according to the invention,

FIG. 2 shows the arrangement of FIG. 1 from above and in partial crosssection,

FIG. 3 in cross section shows a distribution valve incorporated in theembodiment of FIGS. 1 and 2,

FIG. 4 shows a somewhat modified embodiment corresponding to FIG. 2,with the rudder in a deflected position,

FIG. 5 in elevation shows a rudder arrangement according to anotherembodiment,

FIG. 6 shows the arrangement of FIG. 5 from above and in partial crosssection,

FIG. 7 shows an enlarged cross sectional view along the line VII--VII inFIG. 5,

FIG. 8 shows an enlarged detail of the structure at the leading edge ofthe rudder in FIG. 6,

FIG. 9 in schematic elevation shows the leading edge of the rudder inFIG. 8, and

FIG. 10 shows a specific embodiment of a turning mechanism for adjustinga leading edge element of a rudder, e.g. corresponding to the embodimentof FIGS. 6 and 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIGS. 1, 2 and 4 the actual rudder is denoted 5 and this has anassociated rudder stock 9. In FIG. 1 there are shown one part 1 andanother part 2 belonging to the actual ship structure, and in a commonway serving for the journalling of the rudder. FIG. 1 shows a trailingturnable rudder fin 29 which in principle can be of a design known perse. Two forms of auxiliary means for enhancing the steering effect areincorporated in the design of FIGS. 1, 2 and 3. Of primary interest inthe present context is a leading edge profile or profile element 20extending generally vertically along the whole leading edge of rudder 5.Profile element 20 is pivotable about an axis as shown at 25 in thesefigures of drawings; FIG. 2 shows profile element 20 in a neutral,central position, and FIG. 4 shows the element in a deflected angularposition in relation to the rest of rudder 5. As will be explained moreclosely below, this is related to an angular deflection of the actualrudder 5 in FIG. 4 in relation to the centerline of the ship, asindicated at 40 in FIG. 4.

In addition to profile element 20 at the leading edge of rudder 5, thereare also provided nozzles as shown at 30, for discharging seawater flowsthat can additionally serve to assist in increasing the steering effectof the rudder. At this point it is to be noted that nozzle 30 in FIGS. 1and 2 is shown to be pivotable, whereas in the modified embodiment ofFIG. 4 there is a fixed nozzle 30 directing the water flow straightlybackwards from rudder 5. Such fixed are previously known from the patentspecifications referred to above. However, the pivotable nozzles 30 inFIGS. 1 and 2 have a particular and advantageous combined effecttogether with profile element 20.

A pump (not shown) located in the ship itself, serves to furnishseawater at a pressure to rudder 5. For this purpose the rudder stock 9has an internal passageway 9A communicating with a chamber B through aswivel device 3, so that seawater can be transferred from the pump inthe ship to rudder 5. In the rudder there are provided ducts as shown at11 and 12 for the further conveying of seawater to nozzle 30 and to theregion at profile element 20, respectively.

In order to distribute the seawater supply from above through passageway9A in rudder stock 9, there is provided a valve 10 that makes itpossible to adjust the proportion of seawater supply to duct 11 and duct12; a rotatable valve body 15 in valve 10 being adapted to be setangularly by means of a spindle 10A extending up through passageway 9Aand possibly into the ship hull 1 concerned.

Like FIG. 1, FIG. 2 shows a setting of distribution valve 10 so thatvalve body 15 blocks a forward valve opening 14, but opens for seawaterout through a valve opening 13 to duct 11 for water supply to nozzlemeans 30. In FIG. 2 this is shown at various angular positions, of whichtwo are indicated 30' and 30".

FIG. 3 shows another angular setting, i.e. with valve body 15 in frontof opening 13 so that this opening is blocked. Accordingly seawater willexit through opening 14 and therefrom (see FIGS. 2 and 4) more or lessdirectly out into a cavity or a portion of a distribution duct 12 beingformed in the space between the leading edge of the actual rudder 5 andthe inside of profile element 20. In this intermediate space therefor inthe neutral position of the profile element as shown in FIG. 2, therewill exist two laterally directed and slot-shaped distribution ducts 12Aand 12B that communicate with the common cavity 12 centrally in therudder and preferably closely associated with valve 10.

Finally FIG. 4 shows a position of valve body 15 making possible outflowof seawater through both openings 13 and 14, so that nozzle means 30 aswell as profile element 20 with associated active distribution duct 12B,contribute to an enhanced steering effect.

Of essential importance, in a deflected position as shown in FIG. 4, isthe extra seawater flow that is indicated at 12X and coming just fromdistribution duct 12B in the space between the leading edge of rudder 5and profile element 20. This additional flow 12X gives a stronglyincreased lift at the adjacent rudder side, that substantially as awhole contributes to this desired effect. On the other hand at theopposite side, distribution duct 12A is essentially blocked since theend portion of lateral part 20A of profile element 20 engages theadjacent surface 5A of rudder 5. Thus, it is seen that distributionducts 12A and 12B, at either side of the rudder, are adjustable inconsequence of the angular adjustability of profile element 20.

As shown quite schematically in FIG. 2, profile element 20 can beadjusted angularly by means of a turning mechanism comprising e.g. ahydraulic cylinder 21 in association with rudder stock 9, a manoeuvrerod 22 and a turning lever 23 attached to the profile element, wherebythe outer end of rod 22 preferably is articulated to arm 23. Rod 22 andarm 23 are also shown in FIG. 1. Moreover, FIG. 1 shows another rod 24at the upper edge of rudder 5, for angular adjustment of rudder fin 29.Accordingly the angular movement of profile element 20 and fin 29 can becoordinated.

In this connection, it is an advantageous embodiment to build nozzle 30and rudder fin 29 together, so that the nozzle will have an angularmovement corresponding to the angular adjustment of fin 29. FIG. 2illustrates various possible angular positions in that respect, wherebyarrows 30' and 30" as already mentioned above, represent two angularpositions of nozzle 30 and thereby rudder fin 29. In FIG. 2 there arealso shown two nozzle positions laterally at right angles and besides aneutral position with nozzles (30A-B) straight rearwards. It is obviousthat the coordination of the profile element's angular position and thenozzle direction is decisive for a combined optimal effect of theseauxiliary means.

FIG. 1 also shows a particular design of nozzle 30, where the nozzle issubdivided into an upper nozzle outlet 30A and a lower nozzle outlet30B, these converging here towards one another and with an intermediatesplitting element 30C receiving the water flow from duct 11. At thejunction from this at the trailing edge of the actual rudder 5, to fin29 with nozzle 30, there must apparently be provided some form of swivelconnection. Such swivels are known in various embodiments and will notbe more closely described in this context.

The manoeuvring of a ship being provided with a rudder arrangement asdescribed here, in the usual way, will be controlled from operatingmeans or apparatus at the bridge of the ship, for example wherebycontrol and steering systems based on electronics and/or hydraulics in aknown manner can be utilized in a complete system. In this way there canbe provided for steering or regulation of the required pump (e.g.ballast pump), distribution valve, angular setting of profile elementand possibly flap or fin at the trailing edge of the rudder concerned.

The steering can be arranged so that e.g. upon a command from the bridgefor a starboard turn of the ship, the profile element will remain at thesame angle until the rudder deflection has attained e.g. 10°, which cancorrespond to a situation where the distribution duct at one side willbe blocked and the whole water flow be directed to the port side of therudder. Then there will be generated a reduced pressure at the port sideof the rudder compared to the starboard side thereof and the rudder willbe pushed in the port direction. Thus, the ship will turn starboard.Upon further angular movement of the rudder in the same direction, theprofile element will maintain the same setting in relation to the rudderand will follow the turning movement thereof.

According to the embodiment of FIGS. 1 and 2, a seawater flow can at thesame time be led rearwards through the duct in the rudder, whereby thesplitting element 30C in nozzle 30 (FIG. 1) can also have a venturieffect that further increases the thruster force exerted by nozzle 30.The moveability of the output nozzles at the trailing edge of the rudderflap has particular interest at small rudder deflections, in particularthereby that the nozzle angle can be set at up to 90° in relation to thelongitudinal axis of the rudder. Together with the rudder flap or finthe nozzles can be brought to have twice as large an angular deflectionas the rudder itself.

In the second embodiment of the invention, as illustrated in FIG. 5 andthe subsequent figures of drawings, several of the same structuralelements are incorporated as described above with reference to FIGS.1-4. Thus, FIG. 5 shows parts 41 and 42 of the actual ship structure,that as such is also indicated at 44. At the trailing edge of rudder 45there is mounted a pivotable rudder fin 59 and the rudder stock is shownat 49. At the leading edge of the rudder there is provided a profileelement 60 that is preferably angularly adjustable, as is profileelement 20 in the embodiment discussed above. A nozzle 70 is shownrelatively schematically as an integrated part of the rudder fin 59 inFIG. 5 and likewise in FIG. 6.

In this embodiment there is not shown any distribution valvecorresponding to valve 10 in FIGS. 1-4, but on the other hand there areshown at 63 and 64 two supply pipes for water from the ship's pump orpumps, whereby the distribution of water between supplies 63 and 64takes place by suitable means in the ship itself. The two water flowsare then led further downwards in two separate passageways in the formof annular spaces outside and surrounding the rudder stock 49. As willalso appear from the cross sectional illustrations in FIGS. 6 and 7 theannular passageway around the rudder stock 49 is subdivided into twopassages 53 and 54 by means of axially extending partitions 49A and 49B.Advantageously these can be attached to or formed as integrated parts ofthe actual rudder stock 49. At their radially outer edges, partitions49A and 49B engage the inside of the surrounding cylindrical housing 50and during turning movements of rudder 45 there will be maintainedequally large flow cross sections in the two passages 53 and 54. Asshown somewhat schematically in FIG. 5 there is incorporated a swiveldevice 49S between ship part 41 and rudder 45, so that flow passages 53and 54 can extend without hindrance downwards within the rudder atturning movements thereof in relation to the ship. FIG. 7 in thisconnection illustrates possible angular deflections of the rudder, i.e.with 45° to either side in relation to the central position. Openings 51and 52 out through housing 45 lead to respective ducts, being alsodenoted 51 and 52, respectively, in FIGS. 5 and 6. The preferred form ofwater supply being here explained with reference to FIGS. 5, 6 and 7,among other things involves the advantage that it is more simple thanthe design of FIGS. 1-4, since the distribution valve described therein,is not required.

Distribution duct 51 carries seawater to nozzle means 70 through aswivel 59S at the junction or hinge region between the main part 45 ofthe rudder and the rudder fin 59. Besides, in FIG. 5 there is shown avertical distribution duct 57C branched off from duct 51 and providedwith slots or nozzles 57A and 57B at either side of the rudder, fordischarging partial flows of seawater to one or the other rudder side45A or 45B. At the top of distribution duct 57C for this purpose therecan be provided a valve (not shown) to serve for controlling thesepartial flows to the desired rudder side of surface. An additional formof such partial flows of water can also be directed out from nozzles 58Aand 58B as shown in FIG. 6. The outlet opening from these nozzles orslots 58A and 58B can be located in a space between the leading edge ofrudder fin 59 and the trailing edge of the actual rudder 45.

The additional water flows at the trailing portions of rudder 45 as justreferred to, are normally considered to be of relatively subordinatesignificance compared to an additional flow as indicated with arrow 52Xin connection with profile element 60 at the leading edge of the rudder.In contrast to profile element 20 in FIGS. 1-4, the interior of profileelement 60 in the first place is provided with a vertical distributionduct 62 and in association therewith laterally directed ducts or slots52A and 52B at either side. The water flow 52X being indicated in FIG. 6thus can be obtained by outflow of seawater through distribution duct52B. At the top of profile element 60 as schematically illustrated inFIG. 5, there is provided a swivel 62S for interconnecting ducts 52 and62 with a possibility of a certain angular rotation of profile element60 in relation to rudder 45 itself.

FIGS. 8 and 9 illustrate somewhat more in detail the structure foradjusting or controlling the water outflow to one or the other side fromprofile element 60. In FIG. 8 this is shown in an angular position whereduct 52A is blocked, whereas duct 52B is active for discharging theadditional flow 52X referred to above, along the adjacent surface ofrudder 45. A control valve is provided in the form of tube elements 62and 63 surrounding the vertical pivot axis for element 60. The innertube element 62, surrounding the above vertical distribution duct, hastwo longitudinal, vertical slot openings 62A and 62B that can have amutual angular displacement of about 90°. The exterior tube element 63has corresponding longitudinal Slot openings 63A and 63B with a somewhatlarger mutual angular displacement than between openings 62A and 62B.Tube 63 is securely anchored within profile element 60 and will rotatetogether with that element. Tube 62 however, is stationary in relationto rudder 45. Accordingly an angular movement of profile element 60 willhave influence on the water outflow from the duct within tube 62 outthrough the respective more or less coinciding slots and thereby to oneor the other of outflow ducts 52A and 52B.

In accordance with the explanation above the inner tube element 52 canbe connected to a projecting upper part of rudder 45, as will be seenfrom FIG. 5, and can rest on a swivel at the lower end. The outer tubeelement 63 can be considered to constitute a rudder stock for profileelement 60 and in addition to be incorporated as a component in thecontrol valve described, with bearing points at the upper and the lowerends respectively, where a swivel can be provided.

Whereas FIGS. 5-9 have not comprised any turning mechanism for theangular movement of profile element 60, FIG. 10 in a simplified andschematic manner illustrates a design for that purpose. The turningmechanism of FIG. 10 is rotationally driven by drive means generallydenoted 61 associated with the actual rudder stock, such as by a threador worm gear transmission to two manoeuvre rods 66A and 66B which e.g.in the form of telescopical extension and articulation to profileelement 60 at their outer ends, can provide for the desired angularsetting of the profile element about the axis or duct 62.

The embodiments described above make possible relatively simple designsof the rudder parts, in particular when considered in relation to theadvanced functions being obtained. The embodiments of FIGS. 5-9 alsolead to a reduced flow resistance for the seawater supply to the variousauxiliary means serving to enhance the steering effect of the rudder. Itwill be possible to manufacture the profile element and the rear fin ofthe rudder as separate units and likewise the swivel-sleeve arrangementaround the rudder stock in FIGS. 5, 6 and 7 can be assembled as aseparate unit for mounting around the rudder stock through the adjacenthull part. Such embodiments can have much commercial significance alsobecause it can be possible to convert conventional rudders to moresophisticated rudders with an enhanced steering effect, as provided forby this invention.

It will be realized that the embodiments here described with referenceto the drawings, are shown and explained in part in a relativelysimplified and schematic form, for the purpose of primarily to explainthe principles according to the invention. At many points in the designsdescribed it will be possible to make modifications and add structuralelements while maintaining the basic solutions being presented by theinvention.

What is claimed is:
 1. An apparatus for enhancing the steering effect ofship rudders comprising;rudder with at least one passageway forsupplying seawater to ducts in the rudder, and a profile elementextending substantially vertically at a leading edge of the rudder andbeing pivotable about a generally vertical axis, wherein distributionducts formed between the profile element and the rudder communicate withsaid passageway and are adjustable so that relative movement of theprofile element and the rudder causes seawater to be discharged alongthe rudder surface facing abaft.
 2. An apparatus according to claim 1,further comprising a turning mechanism associated with a rudderstock,wherein the profile element is angularly adjustable about saidvertical axis by said turning mechanism, said turning mechanismcomprising at least one manoeuvre rod one end of which is directly orindirectly articulated to the profile element.
 3. An apparatus accordingto claim 2,wherein the profile element comprises internal connectionducts which form a central duct that leads out to either side of theprofile element for discharging seawater adjacent to outer portions ofthe profile element and distributes seawater to either one or the otherside depending upon an angular position of the profile element withrespect to said vertical axis.
 4. An apparatus according to claim 3,further comprising a control valve for controlling distribution ofseawater to either one or the other side of the profile element,saidcontrol valve comprising first and second tubular elements, said tubularelements being concentric about said vertical axis, said first elementbeing rotationally connected to the rudder, said second element beingrotationally connected to the profile element, and both elements havingrespective pairs of cooperating, axially extending slot openings formingsubstantially open and substantially closed flow openings depending uponthe angular position of the profile element, whereby each of the slotopenings in the second tubular element communicates with a separate oneof the distribution ducts.
 5. An apparatus according to claim 1,whereinthe profile element comprises internal connection ducts which form acentral duct that leads out to either side of the profile element fordischarging seawater adjacent to outer portions of the profile elementand distributes seawater to either one or the other side depending uponan angular position of the profile element with respect to said verticalaxis.
 6. An apparatus according to claim 5, further comprising a controlvalve for controlling distribution of seawater to either one or theother side of the profile element,said control valve comprising firstand second tubular elements, said tubular elements being concentricabout said vertical axis, said first element being rotationallyconnected to the rudder, said second element being rotationallyconnected to the profile element, and both elements having respectivepairs of cooperating, axially extending slot openings formingsubstantially open and substantially closed flow openings depending uponthe angular position of the profile element, whereby each of the slotopenings in the second tubular element communicates with a separate oneof the distribution ducts.
 7. An apparatus according to any one ofclaims 1, wherein a space between the leading edge of the rudder and theinside of the profile element constitutes said distribution ducts.
 8. Anapparatus according to claim 1, further comprising:a nozzle with anadjustable angle provided at a trailing edge of the rudder and adaptedto discharge seawater supplied through an associated duct, through therudder.
 9. An apparatus according to claim 8, wherein the nozzle isincorporated in a rear, pivotable rudder fin.
 10. An apparatus accordingto claim 9,wherein said passageway extends in part internally in arudder stock and a distribution valve for supplying water to the profileelement and the nozzle, is arranged axially in the extension of therudder stock.
 11. An apparatus according to claim 10,wherein thedistribution valve comprises a rotatable valve body which depending uponits rotational position is adapted to substantially block at least oneof two valve openings that communicate with a duct to the nozzle on onehand, and the distribution ducts of the profile element on the otherhand.
 12. An apparatus according to claim 8,wherein said passagewayextends in part internally in a rudder stock and a distribution valvefor supplying water to the profile element and the nozzle, is arrangedaxially in the extension of the rudder stock.
 13. An apparatus accordingto claim 12,wherein the distribution valve comprises a rotatable valvebody which depending upon its rotational position is adapted tosubstantially block at least one of two valve openings that communicatewith a duct to the nozzle on one hand, and the distribution ducts of theprofile element on the other hand.
 14. An apparatus according to claim1,wherein said passageway has in part the form of an annular spacesurrounding a rudder stock and a swivel device is provided for arotational connection between a ship hull and the rudder.
 15. Anapparatus according to claim 14, wherein the annular space is subdividedinto two passages of which one communicates with a duct leading to thenozzle device and the other communicates with a duct leading to theprofile element.
 16. An apparatus according to claim 2,wherein a spacebetween the leading edge of the rudder and the inside of the profileelement constitutes said distribution ducts.
 17. An apparatus accordingto claim 2, further comprising:a nozzle with an adjustable angleprovided at a trailing edge of the rudder and adapted to dischargeseawater supplied through an associated duct, through the rudder.
 18. Anapparatus according to claim 17,wherein the nozzle is incorporated in arear, pivotable rudder fin.
 19. An apparatus according to claim2,wherein said passageway has in part the form of an annular spacesurrounding a rudder stock and a swivel device is provided for arotational connection between a ship hull and the rudder.
 20. Anapparatus according to claim 19,wherein the annular space is subdividedinto two passages of which one communicates with a duct leading to thenozzle and the other communicates with a duct leading to the profileelement.
 21. A method for enhancing steering effect of a ship rudder,comprising the steps of:supplying seawater to a rudder duct; pivoting aprofile element relative to said rudder duct to thereby direct saidseawater along a surface of said rudder.
 22. The method according toclaim 21, further comprising the steps of:directing seawater from saidrudder duct to a nozzle provided at a trailing edge of said rudder; anddischarging seawater through said nozzle, said nozzle having anadjustable angle.